Strengthened glass and methods for making using differential chemistry

ABSTRACT

Chemically strengthened glass and a method for making utilizing differential chemistry are provided. The method includes providing a substrate having a glass chemical structure. Host alkali ions are situated in the chemical structure. The substrate has a treatment-rich volume and a treatment-poor volume located as opposed to each other in the substrate. The method also includes providing an exchange medium characterized by including a composition associated with an ion exchange rate of invading alkali ions having an average ionic radius that is larger than an average ionic radius of the host alkali ions. The method also includes providing a modified exchange medium including a modified composition associated with a modified ion exchange rate of the invading alkali ions. The method also includes applying the exchange mediums and conducting ion exchange to produce the strengthened substrate.

PRIORITY

This application claims priority to U.S. Provisional Application No.61/710,139, entitled “Strengthened Glass and Curvature Control” byPatrick K. Kreski filed on Oct. 5, 20112, which is incorporated hereinby reference in its entirety.

BACKGROUND

Chemical strengthening of glass, also called ion-exchange strengtheningor chemical tempering, is a technique to strengthen a prepared glassarticle by increasing compression within the glass itself. It generallyinvolves introducing larger alkali ions into the glass chemicalstructure, to replace smaller alkali ions present in the structure. Acommon implementation of chemical strengthening in glass occurs throughthe exchange of sodium ions, having a relatively smaller ionic radius,with potassium ions, having a relatively larger ionic radius bysubmerging a glass substrate containing sodium ions in a bath containingmolten potassium salts.

Chemical strengthening is often utilized to increase compression inorder to increase strength, abrasion resistance, and/or thermal shockresistance into a glass article. The increased compression can beintroduced to various depths in the glass and is often implementedwithin a surface layer. Chemical strengthening is commonly utilized fortreating flat glass. But it may also be used for treating non-flat glassarticles, such as cylinders and other shapes of greater geometriccomplexity.

Flat glass is commonly manufactured by a number of known techniques.These include the float glass method and drawing methods, such as thefusion down-draw method and the slot draw method. However, a preparedflat glass article may have variations in its chemical compositionand/or structure at different locations in the glass. For example, flatglass that is manufactured by the float glass technique is oftenprepared by spreading softened glass material on a molten metal surfacesuch as tin. The glass is then cooled to form a solid, flat glass. As aresult, the prepared flat glass often contains a greater amount of tinon the side that was nearer the molten tin and the concentration of tinis commonly greater near the surface of that side.

Chemical strengthening is often used to treat glass having variations inchemical composition and/or structure at different locations in theglass. The variations produce locations that are treatment-rich ortreatment-poor relative to each other for ion exchange and/orcompression development in chemical strengthening. When chemicalstrengthening is used to treat such glass, the introduced compressivestress is often not uniformly distributed. This may introduce a bendingmoment and subsequent induced curvature in a glass article treated bychemical strengthening, particularly for glass articles having a widthof less than 3 mm. The induced curvature is often undesirable. Inducedcurvature is especially problematic in manufacturing thin flat glassarticles according to manufacturing specifications that include theenhanced physical properties associated with chemical strengthening, butwithout induced curvature. For example, glass used in manufacturedelectronic articles, such as displays for “smart” phones, often requiresglass that is uniformly flat and high in strength and in abrasionresistance.

For a thin, flat glass article, such as an article having two majorsurfaces, the non-equivalence of interdiffusion of invading alkali ionsand/or compression generation properties between the major surfaces ofthe flat glass substrate after chemical strengthening commonly often hasan effect, such that a local force times the distance from the mid-planeof a glass article is not equivalent when summed from the treatment-poorsurface to the mid-plane and from the treatment-rich surface to themid-plane. Thus the net bending moment about the mid-plane is non-zero(i.e., there is a non-zero net bending moment of the stress about themid-plane). As a result, bending stresses are generated. For glassarticles of thin cross-section, these bending stresses generatedeflection of the glass article from flat. That is, thin, chemicallystrengthened glasses manufactured by the float process often exhibitmeasurable curvature after chemical strengthening. The direction ofcurvature is often concave on the poor surface and convex on the richsurface.

In recent years, various types of efforts have attempted to overcome theproblem of induced curvature that is associated with the chemicalstrengthening of glass. One approach involves grinding and polishing aprepared glass prior to chemical strengthening. The grinding andpolishing is performed to remove those parts of a glass having adifferent chemical composition and/or structure. An example of thisapproach is grinding and polishing a flat glass made by the float methodto remove the surface layer(s) containing a significant amount of tin.However, grinding and polishing the float glass introduces abrasions andmay introduce other physical defects, in addition to the added time andexpense associated with performing the grinding and polishing. Otherapproaches have involved secondary chemical treatments of prepared glassdone prior to chemical strengthening. The secondary chemical treatmentsare utilized in an attempt to address differences chemical compositionand/or structure at different locations in the glass. However secondarychemical treatments can alter the physical properties of the glass andotherwise degrade a glass produced through subsequent chemicalstrengthening. Also, like grinding and polishing, secondary chemicaltreatments involve the time and expense of an extra processing step thatis done prior to chemical strengthening.

Given the foregoing, chemically strengthened glass and methods formaking chemically strengthened glass are desired in which thestrengthened glass has reduced induced curvature. It is also desiredthat the strengthened glass not have the drawbacks associated withgrinding and polishing or secondary chemical treatment(s) applied inprior methods associated with the chemical strengthening of the glass.It is also desired that the strengthened glass have the improvedphysical properties of chemically strengthened glass, such as higherstrength, higher abrasion resistance, and/or higher thermal shockresistance.

SUMMARY

This summary is provided to introduce a selection of concepts. Theseconcepts are further described below in the detailed description inconjunction with the accompanying drawings. This summary is not intendedto identify key features or essential features of the claimed subjectmatter, nor is this summary intended as an aid in determining the scopeof the claimed subject matter.

According to an implementation, there is a method for making astrengthened substrate. The method may include providing a substrate.The substrate may be characterized by having a glass chemical structure.The glass chemical structure may include host alkali ions having anaverage ionic radius situated in the glass chemical structure. Thesubstrate may have dimensional volumes including a treatment-rich volumeand a treatment-poor volume. The volumes may be located as opposed toeach other in the substrate. The method may also include providing anexchange medium. The exchange medium may be characterized by including acomposition including invading alkali ions having an average ionicradius that is larger than the average ionic radius of the host alkaliions. The composition may be associated with an ion exchange rate of theinvading alkali ions. The method may also include providing a modifiedexchange medium characterized by including a modified compositionincluding invading alkali ions. The modified composition may beassociated with a modified ion exchange rate that is slower than the ionexchange rate. The method may include applying the modified exchangemedium to a surface of the treatment-rich volume. The method may alsoinclude applying the exchange medium to a surface of the treatment-poorvolume. The method may also include conducting ion exchange whileapplying at least one of the exchange medium and the modified exchangemedium to produce the strengthened substrate.

According to another implementation, there is an article of manufacture.The article may include a chemically strengthened substratecharacterized by having a glass chemical structure including alkali ionssituated in the glass chemical structure. The substrate may havedimensional volumes including a treatment-rich volume including a richsurface of the substrate. The volumes may also include a treatment-poorvolume including a poor surface of the substrate and characterized byhaving a variation from the treatment-rich volume in at least one of achemical composition and a chemical structure. The volumes may alsoinclude a bulk volume, within the substrate, that may be adjacent atleast one of the treatment-rich volume and the treatment-poor volume. Aconcentration of metal may be in at least one of the treatment-poorvolume and the treatment-rich volume. The concentration of metal may be≧about 0.4 mole % higher than a concentration of the metal in the bulkvolume. A concentration of the metal may be higher in the treatment-poorvolume than a concentration of the metal in the treatment-rich volume. Aconcentration of alkali ions may be in a diffusion depth of at least oneof the treatment-rich volume and the treatment-poor volume. Theconcentration of alkali ions may be ≦about 0.5 mole % higher than aconcentration of the alkali ions in the bulk volume.

According to another implementation, there is an article of manufacture.The article may include a chemically strengthened substrate. Thechemically strengthened substrate may be made by a process includingproviding a substrate. The substrate may be characterized by having aglass chemical structure comprising host alkali ions having an averageionic radius situated in the glass chemical structure. The substrate mayhave dimensional volumes including a treatment-rich volume and atreatment-poor volume. The volumes may be located as opposed to eachother in the substrate. The process may also include providing anexchange medium. The exchange medium may be characterized by including acomposition including invading alkali ions having an average ionicradius that is larger than the average ionic radius of the host alkaliions. The composition may be associated with an ion exchange rate of theinvading alkali ions. The process may also include providing a modifiedexchange medium. The modified exchange medium may include a modifiedcomposition including invading alkali ions. The modified composition maybe associated with a modified ion exchange rate of invading alkali ionsthat is slower than the ion exchange rate. The process may also includeapplying the modified exchange medium to a surface of the treatment-richvolume. The process may also include applying the exchange medium to asurface of the treatment-poor volume. The process may also includeconducting ion exchange while applying at least one of the exchangemedium and the modified exchange medium to produce the strengthenedsubstrate.

The above summary is not intended to describe each embodiment or everyimplementation. Further features, their nature and various advantagesare described in the accompanying drawings and the following detaileddescription of the examples and embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one or more embodiments describedherein and, together with the description, explain these embodiments. Inaddition, it should be understood that the drawings are presented forexample purposes only. In the drawings:

FIG. 1 is a flowchart illustrating an exemplary overview of animplementation described herein;

FIG. 2 is a graph showing properties of exemplary strengthenedsubstrates made utilizing exchange mediums including a monovalentadditive;

FIG. 3 is a graph showing properties of exemplary strengthenedsubstrates made utilizing exchange mediums including a divalentadditive; and

FIG. 4 is a flowchart illustrating an exemplary process for making astrengthened substrate.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings.The same reference numbers in different drawings may identify the sameor similar elements.

Overview

The present invention is useful for making chemically strengthenedglass, and has been found to be particularly advantageous for makingchemically strengthened glass having reduced induced curvature. Achemically strengthened glass, according to the principles of theinvention, does not have the drawbacks associated with grinding andpolishing or secondary chemical treatment(s) when done prior to chemicalstrengthening. While the present invention is not necessarily limited tosuch applications, various aspects of the invention are appreciatedthrough a discussion of various examples using this context.

FIG. 1 is a flowchart illustrating an exemplary overview of animplementation described herein. Assume that a glass substrate hasvariations in its chemical composition and/or chemical structure atdifferent locations or “volumes” in the glass. One type of variation hasa chemical composition and/or chemical structure that is more readilytreated by chemical strengthening and is a “treatment-rich” volume.Another type of variation has a chemical composition and/or chemicalstructure that is less readily treated by chemical strengthening and isa “treatment-poor” volume. The term “treatment-rich volume” refers to avolume of a glass substrate which exhibits faster alkali ioninterdiffusion and/or greater compression development during chemicalstrengthening relative to a “treatment-poor volume” under equivalentchemical strengthening conditions applied to the glass substrate. Avolume may occur at a surface of a substrate, or in a space or layerbeneath the surface. A treatment-rich volume or treatment-poor volumemay be a surface layer of a glass substrate in which the diffusion ofinvading alkali ions extends to a given “diffusion depth” from thesurface, also called a penetration depth or a diffusion layer. Inchemical strengthening, a portion of the diffusion depth is incompressive stress, called case depth. Case depth is the width of thediffusion layer that is in compressive stress in a specimen.

Different exchange mediums may be utilized in performing the chemicalstrengthening of the treatment-rich volume and the treatment-poorvolume. Assume also that the different exchange mediums may bedistinguished by including different compositions including invadingalkali ions within the different exchange mediums. The compositions maybe varied in many ways to affect the ion exchange rate associated with acomposition in an exchange medium. One type of variation may include oneor more additives to at least one of the compositions. Other variationsmay also be utilized such as by utilizing different solvents, differentspecies of invading ions, etc. A variation in a composition may decreaseor increase an ion exchange rate associated with a composition in anexchange medium.

As shown in FIG. 1, at step 102, a glass substrate is provided withdifferent volumes, a treatment-rich volume and a treatment-poor volume.At step 104, an exchange medium, including a composition includinginvading alkali ions, is applied to a surface of a treatment-poorvolume. At step 106, a modified exchange medium, including a modifiedcomposition, is applied to a surface of a treatment rich volume in theglass substrate. The modified composition includes, for example, anadditive, such as ions that compete with the invading alkali ions. Theadditive has the effect of “poisoning” the modified exchange medium,thus lowering its ion exchange rate of invading alkali ions. Thus, themodified ion exchange rate of the modified exchange medium is slowerthan the ion exchange rate of invading alkali ions in the exchangemedium applied in step 104.

While the exchange mediums are applied, in step 104 and step 106,chemical strengthening proceeds to produce a strengthened substrate inwhich the induced curvature has been reduced or nullified through theapplication of the different exchange mediums to the different volumes.Without wishing to be bound by any particular theory, it appears thatthe slower ion exchange rate of invading alkali ions in the modifiedexchange medium applied to the treatment-rich volume offsets thedifference in ion-exchangeability between the treatment-rich andtreatment-poor volumes, thus reducing or nullifying induced curvaturethat would otherwise result from chemical strengthening of the glasssubstrate.

For simplicity and illustrative purposes, the present invention isdescribed by referring mainly to embodiments, principles and examplesthereof. In the following description, numerous specific details are setforth in order to provide a thorough understanding of the examples. Itis readily apparent however, that the embodiments may be practicedwithout limitation to these specific details. In other instances, someembodiments have not been described in detail so as not to unnecessarilyobscure the description. Furthermore, different embodiments aredescribed below. The embodiments may be used or performed together indifferent combinations.

The operation and effects of certain embodiments can be more fullyappreciated from the examples, as described below. The embodiments onwhich these examples are based are representative only. The selection ofthese embodiments to illustrate the principles of the invention does notindicate that materials, components, reactants, conditions, techniques,configurations and designs, etc. which are not described in the examplesare not suitable for use, or that subject matter not described in theexamples is excluded from the scope of the appended claims or theirequivalents. The significance of the examples may be better understoodby comparing the results obtained therefrom with potential results whichmay be obtained from tests or trials that may be, or may have been,designed to serve as controlled experiments and to provide a basis forcomparison.

As used herein, the terms “based on”, “comprises”, “comprising”,“includes”, “including”, “has”, “having” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. Further, unless expressly stated to the contrary,“or” refers to an inclusive or and not to an exclusive or. For example,a condition A or B is satisfied by any one of the following: A is true(or present) and B is false (or not present), A is false (or notpresent) and B is true (or present), and both A and B is true (orpresent). Also, use of the “a” or “an” is employed to describe elementsand components. This is done merely for convenience and to give ageneral sense of the description. This description should be read toinclude one or at least one and the singular also includes the pluralunless it is obvious that it is meant otherwise.

The meaning of abbreviations and certain terms used herein is asfollows: “mm” means millimeter(s), “μm” means micrometer(s) ormicron(s), “g” means gram(s), “mg” means milligram(s), “μg” meansmicrogram(s), “L” means liter(s), “mL” means milliliter(s), “cc” meanscubic centimeter(s), “cc/g” means cubic centimeters per gram, “mol”means mole(s), “mmol” means millimole(s), “wt %” means percent by weightand “mol %” means percent by mole.

Exemplary Substrate Glasses

As used herein a “glass substrate” may comprise any kind ofion-exchangeable glass. Examples of such glass include soda-limesilicate glass, alkali aluminosilicate glass or alkalialuminoborosilicate glass, though other glass compositions arecontemplated including glasses where glass forming components are freeof silica, such as boron oxide (borate), phosphorous oxide (phosphate),aluminum oxide (aluminate), etc. As used herein, “ion exchangeable”means that a glass is capable of exchanging alkali ion located in theglass structure of the glass (i.e., “host alkali ions”), such as at ornear the surface of the substrate, with larger alkali ions (i.e.,“invading alkali ions”) from an exchange medium that may be a liquid,solid or gas. An “ion exchange rate” refers to an amount of invadingions entering a substrate over a period of time. A glass may havechemical composition and/or chemical structure variations at differentlocations or “volumes” in the glass. An example of chemical compositionvariation is an excess of metal, such as metal ions or other forms ofmetal and may include a metal species, such as tin or lead. An exampleis metal that remains in a flat glass made by a float glass method, suchas tin. An example of chemical structure variation is the presence of anelement in the glass in which the element may have different valencesthroughout different volumes, such as tin present in Sn²⁺ and Sn⁴⁺valences in the different volumes. In this example, the different formsof tin form different chemical structures in the different volumes.

Exemplary embodiments of substrate glasses include silicate glasses,such as soda-lime silicate glass or sodium aluminosilicate glass thatincludes alumina, at least one alkali metal and, in some embodiments,greater than 50 mol % SiO₂, in other embodiments at least 58 mol % SiO₂,and in still other embodiments at least 60 mol % SiO₂.

Exemplary Strengthened Glasses

Exemplary embodiments of chemically strengthened glasses includesoda-lime silicate glass and sodium aluminosilicate glass which arestrengthened, such as, in potassium nitrate salt baths. Chemicalstrengthening may be performed at various temperatures, such as attemperatures above about 400° C., preferably about 430° C., and with ionexchange durations of about 1-24 hours. The zone of compressive stressoccurs, for example, within a diffusion depth of about 5, 10, 20, 30,40, 50, 60, 70, 80, 90, 100 or 125 to about 150 μm of a surface of asubstrate glass. According to an exemplary embodiment, compressivestress in a strengthened glass is greatest at a surface (i.e., a“surface compression”) of the glass and the level of compressive stressfollows a gradient extending downward from the surface through a casedepth in the strengthened glass. In exemplary embodiments, the amount ofsurface compression may be up to about 800 MPa or higher in strengthenedsoda-lime silicate glass and up to about 1200 MPa or higher inaluminosilicate glass. In some exemplary embodiments, surfacecompression is about 200-650 MPa in strengthened soda-lime silicateglass and about 300-850 MPa in aluminosilicate glass. In other exemplaryembodiments, surface compression is about 400-600 MPa in strengthenedsoda-lime silicate glass and about 600-800 MPa in aluminosilicate glass.

In some exemplary embodiments, a strengthened silicate glass, such assoda-lime silicate glass or sodium aluminosilicate glass comprisesalumina, at least one alkali metal and, in some embodiments, greaterthan 50 mol % SiO₂, in other embodiments at least 58 mol % SiO₂, and instill other embodiments at least 60 mol % SiO₂. In these embodiments, aLi₂O+Na₂O+K₂O total mol %, such as in a volume associated with adiffusion depth, is at least about 1, 2, 5, 7 or 8-10 mol % and ≦25 mol%, preferably ≦20 mol %, and more preferably ≦about 2, 5, 7, 8, 10, 12,15 or 16-18 mol %.

In another exemplary embodiment, an alkali aluminosilicate glasscomprises, consists essentially of, or consists of: 60-75 mol % SiO₂;5-15 mol % Al₂O₃; 0-12 mol % B₂O₃; 8-21 mol % Na₂O; 0-8 mol % K₂O; 0-15mol % MgO; and 0-3 mol % CaO. In these embodiments, such as in a volumeassociated with a diffusion depth, a Li₂O+Na₂O+K₂O total mol % is atleast about 1, 2, 5, 7 or 8-10 mol % and ≦25 mol %, preferably ≦20 mol%, and more preferably ≦about 2, 5, 7, 8, 10, 12, 15 or 16-18 mol %.

In yet another embodiment, an alkali aluminosilicate glass substratecomprises, consists essentially of, or consists of: 60-70 mol % SiO₂;6-14 mol % Al₂O₃; 0-15 mol % B₂O₃; 0-15 mol % Li₂O; 0-20 mol % Na₂O;0-10 mol % K₂O; 0-15 mol % MgO; 0-10 mol % CaO; 0-5 mol % ZrO₂; 0-2 mol% SnO₂; 0-1 mol % CeO₂; wherein about 1, 2, 5, 7, 8, or 10-12 mol%≦Li₂O+Na₂O+K₂O≦about 2, 5, 7, 8, 10, 12, 15 or 16-20 mol %, such as ina volume associated with a diffusion depth, and 0 mol %≦MgO+CaO≦15 mol%.

In one example embodiment, sodium ions in the substrate glass arereplaced by potassium ions from a molten bath, though other alkali metalions having a larger atomic radius, such as rubidium or cesium, mayreplace smaller alkali metal ions in the glass. Similarly, other alkalimetal salts such as, but not limited to, nitrates, sulfates, halides,and the like may be used in the ion exchange process.

In another example embodiment, a chemically-strengthened glass substratecan have a surface compressive stress of about 200 MPa or more, e.g.,about 300, 400, 500, 600, 700, 800, 900, 1000 or 1500 MPa or more, acase depth of about 5 μm or more (e.g., about 5, 10, 20, 25, 30, 35, 40,45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100 μm or more) and adiffusion depth of about 5 μm or more (e.g., about 5, 10, 20, 25, 30,35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 125 or 150 μmor more).

In another example embodiment, a chemically-strengthened glass substratecan have a higher amount of metal in at least one surface volume orlayer, such as a treatment-rich volume or a treatment-poor volume, thanin a bulk volume adjacent these surface volumes. A concentration ofmetal in at least one of the treatment-poor volume and thetreatment-rich volume may be ≧about 0.4, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0,5.0, 6.0, 8.0, 10.0, 12.0, 15.0, 20 or 25 mol % higher than aconcentration of the metal in the bulk volume. According to anembodiment, a concentration of metal in the treatment-poor volume ishigher than a concentration of the metal in a treatment-rich volume. Acommon example of strengthened glass with variant metal concentrationsin the different volumes is chemically strengthened glass made from aflat glass substrate prepared using a float glass process utilizing tin.

In another example embodiment, a chemically-strengthened glass substratemay have an average concentration of alkali ions (e.g. invading alkaliions and host alkali ions) that is the same or different in a diffusiondepth of a surface volume than in an adjacent volume, such as a bulkvolume. The surface volume may be a treatment-rich volume or atreatment-poor volume in the strengthened glass. The averageconcentration of alkali ions may be the same or different from anaverage concentration of alkali ions in the adjacent volume, such as abulk volume. In one example embodiment, the average concentration ofalkali ions in the diffusion depth of the surface volume is ≦to about0.5 mol % higher than a concentration of the alkali ions in the bulkvolume. In other embodiments, the average concentration of alkali ionsin the diffusion depth of the surface volume is ≦to about 0.4, 0.3, 0.2,0.1 or 0.05 mol % higher, equal to or less than a concentration of thealkali ions in the bulk volume adjacent the surface volume.

Exemplary Exchange Mediums

Exemplary embodiments of a liquid exchange medium which may be utilizedin chemical strengthening include liquid molten salt baths. The moltenliquid baths include invading alkali ions having an average ionic radiusin the alkali metal ion of the molten salt that is larger than anaverage ionic radius of host alkali metal ions in the substrate glassprior to ion exchange. A common example of a liquid molten salt bathincludes potassium nitrate with potassium as the invading alkali ion toreplace sodium and/or lithium host ions in the substrate glass.

Mixed salt blends of invading alkali ions may also be used as liquidexchange mediums. These blends may include salts of different alkalimetals, preferably different alkali metal nitrates. A nitrate melt blendmay include at least two different alkali ions, for example Na and K, oras well Na and Rb. But it is also possible that three or four differentalkali metals are included. Rb ions or Cs ions may be used in chemicalstrengthening. The method according to the embodiment offers the optionto effectively incorporate invading alkali ions into a treated glassarticle having ionic radii that are significantly larger than the radiiof host alkali ions, such as lithium or sodium ions.

Exemplary embodiments of a solid exchange medium which may be utilizedin chemical strengthening include semi-solid pastes that may be appliedto a surface of a glass substrate. The paste includes invading alkaliions from a source such as a salt and at least one rheological agent,such as clay, to suspend the ions in the solid exchange medium. Kaolinis a common example of a rheological agent which may utilized in makinga solid exchange medium. The viscosity of a paste made with kaolin maybe modified with water and other additives to suit an application bywhich the paste is applied to a glass substrate. Water content of apaste may be evaporated prior to application as a solid exchange mediumutilizing a raised high temperature, such as greater than 120° C.Another example of a rheological agent is aluminosilicate fiber. Otherclays and rheological agents are also contemplated.

In addition to liquid and solid exchange mediums, gas exchange mediumsare also contemplated.

Exemplary Chemical Differentiators

An exchange medium may include a composition that affects the ionexchange rate of invading alkali ions in the exchange medium. Thecomposition may increase or decrease the rate of ion exchange ofinvading alkali ions into a substrate. The composition may be modifiedin many different ways to affect the rate of ion exchange associatedwith the exchange medium compared with another exchange medium under amatching or similar set of conditions. For example, the compositions inthe different exchange mediums may be altered by modifying one of thecompositions to include an additive that slows the rate of ion exchange.

Examples of additives which may be utilized to modify the rate of ionexchange associated with an exchange medium include, for example,various salts such as nitrates, sulfates, phosphates, and other saltscontaining smaller-sized alkali ions that are not the invading alkaliions such as sodium, lithium and alkaline earth salts such as those ofcalcium, strontium, or barium. Exemplary species of salts as “poisoning”species to KNO₃ as the primary invading alkali ion salt, for instance,include, but are not limited to NaNO₃, NaCl, LiNO₃, LiCl, and Ca(NO₃)₂.Other substances contributing various ions such as hydrogen or hydroxylions may also be utilized to alter the ion exchange rate of invadingalkali ions from an exchange medium. Other additives are alsocontemplated.

EXAMPLES

The following examples demonstrate methods of making chemicallystrengthened glass utilizing differential chemistry methodology.

Example 1 Example 1 demonstrates the preparation of a chemically

strengthened soda-lime silicate glass having a reduced inducedcurvature. Reference is made to graph 200 in FIG. 2 in the example.Graph 200 shows a differential chemistry methodology. Chemistry of apaste exchange medium is varied by replacing KNO₃, providing invadingalkali ions, with NaNO₃, providing competitive or poisonous ions, in apaste applied to a “rich surface” (i.e., a surface of a treatment-richvolume in a substrate). In graph 200, mol % replacement of KNO₃ by NaNO₃in the paste applied to the rich surface is shown on the abscissa.Deflection for a flat glass having 50 mm span is given in microns on theordinate. Note that for this example, an ideal flatness, (i.e., a nearzero induced curvature) is crossed between 2.4 and 5.0 mol % NaNO₃, asshown by the data plotted in graph 200.

Sample Preparation:

Soda-lime silicate glass coupons, 50 mm×50 mm across and 0.4 mm width,were cut from a mother sheet formed by a tin float glass process. Bothsurfaces (i.e., the treatment-rich surface and the treatment-poorsurface) of the coupons were manually coated with a uniform amount ofKNO₃-containing paste as a solid exchange medium. An equivalent quantityof paste by weight was applied onto each surface. However, on separatecoupons, the paste on the treatment-rich surface had 0.5, 1.0, 2.4, and5.0 mol % of the total KNO₃ in the paste replaced by NaNO₃. After dryingthe pastes on the coupons, all coupons were processed in air at 440° C.for 8 hours to allow for ion-exchange. After the ion-exchange treatment,the coupons were removed, air-cooled, and rinsed with water to removethe paste. A minimum of two coupons were examined for each parameter.

Results:

Coupon deflection after processing was determined from surface profilesmeasured using a non-contact optical profiler. Deflection is thepeak-to-valley height determined along a line drawn between oppositeedge mid-points of the square coupon. Deflection versus replacement ofKNO₃ by NaNO₃ (mol %) is given in graph 200. A positive deflectionmeasurement in graph 200 indicates convex curvature of the rich surface.A negative deflection measurement indicates concave curvature of therich surface.

The replacement of KNO₃ by NaNO₃ (mol %) produced a deflectionmeasurement of less than the 50 micron target for 2.4 and 5.0 mol % inthis example. The ideal flat was crossed between 2.4 and 5.0 mol %replacement NaNO₃. At 2.4 mol % replacement NaNO₃, average deflectionwas 35.4 micron (rich surface convex), average surface compression was374 MPa for the rich surface and 384 MPa for the poor surface, averagecase depth was 14.3 micron for the rich surface and 13.8 micron for thepoor surface.

Comparative sample coupons, which had the KNO₃ paste applied to the richsurface, free of NaNO₃ replacement, and otherwise underwent equivalentprocessing had an average deflection measurement of 42.1 micron (richsurface convex), average surface compression was 427 MPa for the richsurface and 423 MPa for the poor surface, and average case depth was16.0 micron on the rich surface and 15.8 micron on the poor surface. Theaverage deflection of the comparative coupons is represented by thehorizontal dashed line in graph 200 in FIG. 2.

Example 2

Example 2 demonstrates the preparation of a chemically strengthenedsoda-lime silicate glass having a reduced induced curvature. Referenceis made to graph 300 in FIG. 3. Graph 300 shows a differential chemistrymethodology. The chemistry of a paste exchange medium is varied byreplacing KNO₃, providing invading alkali ions, with Ca(NO₃)₂ providingcompetitive or poisonous ions, in a paste applied to a “rich surface”(i.e., a surface of a treatment-treatment rich volume in a substrate).In graph 300, the mol % replacement of KNO₃ by Ca(NO₃)₂ in the pastesapplied to the rich surface are shown on the abscissa. Deflection for aflat glass having 50 mm span is given in microns on the ordinate. Notethat for this example, an ideal flatness, (i.e., a near zero inducedcurvature) is crossed near 5.0 mol % Ca(NO₃)₂, as shown by the dataplotted in graph 300.

Sample Preparation:

Soda-lime silicate glass coupons, 50 mm×50 mm across and 0.4 mm width,were cut from a mother sheet formed by a tin float glass process. Bothsurfaces (i.e., the treatment-rich surface and the treatment-poorsurface) of the coupons were manually coated with a uniform amount ofKNO₃-containing paste as a solid exchange medium. An equivalent quantityof paste by weight was applied onto each surface. However, on separatecoupons, the paste on the treatment-rich surface had 1.0, 5.0, and 10.0mol % of the total KNO₃ in the paste replaced by Ca(NO₃)₂. After dryingthe pastes on the coupons, all coupons were processed in air at 440° C.for 24 hours to allow for ion-exchange. After the ion-exchangetreatment, the coupons were removed, air-cooled, and rinsed with waterto remove the paste. A minimum of two coupons were examined for eachparameter.

Results:

Coupon deflection after processing was determined from surface profilesmeasured using a non-contact optical profiler. Deflection is thepeak-to-valley height determined along a line drawn between oppositeedge mid-points of the square coupon. Deflection versus replacement ofKNO₃ by Ca(NO₃)₂ (mol %) is given in graph 300. A positive deflectionmeasurement in graph 300 indicates convex curvature of the rich surface.A negative deflection measurement indicates concave curvature of therich surface.

The replacement of KNO₃ by Ca(NO₃)₂ (mol %) produced a deflectionmeasurement of less than the 50 micron target for 1.0 and 5.0 mol % inthis example. The ideal flat was crossed near 5.0 mol % replacementCa(NO₃)₂. At 5.0 mol % replacement Ca(NO₃)₂, average deflection was 8.2micron (rich side convex), average surface compression was 333 MPa forthe rich surface and 369 MPa for the poor surface, average case depthwas 23.2 micron for the rich surface and 26.4 micron for the poorsurface.

Comparative sample coupons, which had the KNO₃ paste applied to the richsurface, free of Ca(NO₃)₂ replacement, and otherwise underwentequivalent processing had an average deflection measurement of 157.3micron (rich side convex), average surface compression was 412 MPa forthe rich surface and 379 MPa for the poor surface, and average casedepth was 27.1 micron on the rich surface and 27.9 micron on the poorsurface. The average deflection of the comparative coupons isrepresented by the horizontal dashed line shown in graph 300 in FIG. 3.

FIG. 4 is flowchart illustrating exemplary processes for making astrengthened substrate.

At step 402, a glass substrate is provided having different volumes,such as a “treatment-rich” volume and a “treatment-poor” volume in theglass structure including host alkali ions. The glass may be soda-limesilicate glass or aluminosilicate glass. The volumes may be located, forexample, as opposed to each other in the substrate, and according to anembodiment, may be diametrically opposed. The glass substrate may havevariations in the different volumes, such as a variation in chemicalcomposition and/or chemical structure. An example of a variation inchemical composition is an amount of tin situated in different volumesof the glass. An example of a variation in chemical structure is thepresence of tin in different valences, Sn²⁺ and Sn⁴⁺ in differentvolumes of the glass. A variation in chemical composition and/orchemical structure in the treatment-poor volume may distinguish it fromthe treatment-rich volume.

At step 404, an exchange medium such as a paste having a compositionmade from kaolin and water, and includes KNO₃ providing invading alkaliions that are potassium, is applied to a surface of the treatment-poorvolume.

At step 406, according to an embodiment, a modified exchange mediumincludes a modified composition that comprises Ca(NO₃)₂+KNO₃ that is 95mol. % KNO₃ and 5 mol. % Ca(NO₃)₂ as poisonous or competitive ions, butis otherwise the same paste applied as the exchange medium in Step 404.The modified exchange medium is applied to a surface of thetreatment-rich volume.

At step 408, ion exchange is conducted on the glass substrate with theexchange medium and the modified exchange medium applied to therespective surfaces. According to an embodiment, a net bending moment isabout zero, in a fully strengthened substrate after the exchange mediumand modified exchange mediums have been applied and the compressivestress varies at different locations in at least one of thetreatment-poor volume and the treatment-rich volume of the strengthenedsubstrate.

Although described specifically throughout the entirety of thedisclosure, the representative examples have utility over a wide rangeof applications, and the above discussion is not intended and should notbe construed to be limiting. The terms, descriptions and figures usedherein are set forth by way of illustration only and are not meant aslimitations. Those skilled in the art recognize that many variations arepossible within the spirit and scope of the principles of the invention.While the examples have been described with reference to the figures,those skilled in the art are able to make various modifications to thedescribed examples without departing from the scope of the followingclaims, and their equivalents.

What is claimed is:
 1. A method for making a strengthened substrate, themethod comprising: providing a substrate characterized by having a glasschemical structure including host alkali ions having an average ionicradius situated in the glass chemical structure, and wherein thesubstrate has dimensional volumes including a treatment-rich volume anda treatment-poor volume located as opposed to each other in thesubstrate; providing an exchange medium including a compositionincluding invading alkali ions having an average ionic radius that islarger than the average ionic radius of the host alkali ions, whereinthe composition is associated with an ion exchange rate of the invadingalkali ions; providing a modified exchange medium including a modifiedcomposition including invading alkali ions, wherein the modifiedcomposition is associated with a modified ion exchange rate of theinvading alkali ions, and wherein the modified ion exchange rate isslower than the ion exchange rate; applying the modified exchange mediumto a surface of the treatment-rich volume; applying the exchange mediumto a surface of the treatment-poor volume; and conducting ion exchangewhile applying at least one of the exchange medium and the modifiedexchange medium to produce the strengthened substrate.
 2. The method ofclaim 1, wherein the modified composition includes at least one additiveassociated with reducing the rate of ion exchange of the modifiedexchange medium.
 3. The method of claim 2, wherein the additive includesions associated with competing with the invading alkali ions for accessto the exchange-rich surface during ion exchange.
 4. The method of claim2, wherein the additive includes at least one of monovalent ions anddivalent ions.
 5. The method of claim 2, wherein the additive comprisesat least one of NaNO₃ and Ca(NO₃)₂.
 6. The method of claim 1, whereinthe substrate comprises a variation in at least one of chemicalcomposition and chemical structure in the substrate.
 7. The method ofclaim 6, wherein at least one of the chemical composition and chemicalstructure in the treatment-poor volume is different than in thetreatment-rich volume.
 8. The method of claim 1, wherein a net bendingmoment about mid-plane is about zero in the strengthened substrate. 9.The method of claim 1, wherein a compressive stress varies at differentlocations of the strengthened substrate.
 10. The method of claim 1,wherein conducting ion exchange is performed at a constant temperature.11. The method of claim 1, wherein conducting ion exchange is performedwhile applying about an equal temperature to the treatment-poor volumeand the treatment-rich volume.
 12. The method of claim 1, wherein theexchange medium and the modified exchange medium is one of a liquid, asolid, a gas and mixtures thereof.
 13. The method of claim 1, whereinthe treatment-rich volume and the treatment-poor volume are located asdiametrically opposed in the substrate.
 14. The method of claim 1,wherein the method is one of a continuous process and a batch process.15. The method of claim 1, wherein the substrate comprises one of alkalialuminosilicate glass and soda-lime silicate glass.
 16. The method ofclaim 1, wherein the substrate is flat.
 17. The method of claim 1,wherein the substrate has a width of about 3.0 millimeters or less. 18.An article of manufacture, the article comprising: a chemicallystrengthened substrate characterized by having a glass chemicalstructure including alkali ions situated in the glass chemical structurewherein the substrate has dimensional volumes including a treatment-richvolume including a rich surface of the substrate, a treatment-poorvolume including a poor surface of the substrate and characterized byhaving a variation from the treatment-rich volume in at least one of achemical composition and a chemical structure, and a bulk volume, withinthe substrate, adjacent at least one of the treatment-rich volume andthe treatment-poor volume, wherein a concentration of metal in at leastone of the treatment-poor volume and the treatment-rich volume is ≧about0.4 mole % higher than a concentration of the metal in the bulk volume,wherein a concentration of the metal is higher in the treatment-poorvolume than a concentration of the metal in the treatment-rich volume,and wherein a concentration of alkali ions in a diffusion depth of atleast one of the treatment-rich volume and the treatment-poor volume is≦about 0.5 mole % higher than a concentration of the alkali ions in thebulk volume.
 19. The article of claim 18, wherein at least one of thetreatment-rich volume and the treatment-poor volume has a diffusiondepth of about 5 to 150 μm.
 20. The article of claim 18, wherein thechemically strengthened glass substrate comprises greater than 50 mole %SiO₂.
 21. The article of claim 18, wherein the chemically strengthenedglass substrate comprises about 1 to 25 total mole % of Li₂O+Na₂O+K₂O inthe diffusion depth, wherein the diffusion depth is about 5 to 150 μm.22. An article of manufacture, the article comprising: a chemicallystrengthened substrate made by a process including providing a substratecharacterized by having a glass chemical structure comprising hostalkali ions having an average ionic radius situated in the glasschemical structure, and wherein the substrate has dimensional volumesincluding a treatment-rich volume and a treatment-poor volume located asopposed to each other in the substrate, providing an exchange mediumincluding a composition including invading alkali ions having an averageionic radius that is larger than the average ionic radius of the hostalkali ions wherein the composition is associated with an ion exchangerate of the invading alkali ions, providing a modified exchange mediumincluding a modified composition including invading alkali ions whereinthe modified composition is associated with a modified ion exchange rateof the invading alkali ions and wherein the modified ion exchange rateis slower than the ion exchange rate, applying the modified exchangemedium to a surface of the treatment-rich volume; applying the exchangemedium to a surface of the treatment-poor volume; and conducting ionexchange while applying at least one of the exchange medium and themodified exchange medium to produce the strengthened substrate.